Interactions of polyelectrolytes bearing carboxylate and/or sulfonate groups with Cu(II) and Ni(II)
Introduction
Polyelectrolytes (PEs), polymers with a high concentration of ionic groups or ionogenes, have the ability to chelate or exchange metal ions [1], [2], [3], [4]. This property facilitates their use to recover and/or separate metal ions from aqueous solution. In this context, the PEs are used in water treatment and in hydrometallurgy at both industrial and laboratory scales, for quantitative analytical and recovery procedures [5], [6].
The polyelectrolyte–metal ion interaction can be only electrostatic or include the formation of coordination bonds. The type of interaction depends on the chemical nature (ionization potential and electronic affinity) of the functional groups. The variables that affect the polyion–metal ion interaction are classified in two groups: intrinsic and extrinsic to the polymer. The former includes the polymer structure in terms of composition and geometry, which affects the flexibility of the chains in solution: branches of the chain, chemical nature of the functional groups, and their distribution at the polymer chain, etc. The second group includes the charge and type of the metal ion, pH of the solution, ionic strength, temperature, and dielectric constant of the medium [7].
The study of the polyelectrolyte–metal ion interaction can be carried out by different techniques such as potentiometry [8], [9], [10], [11], [12], [13], spectrophotometry [9], [11], [12], [14], [15], [16], [17], [18], viscosimetry [11], [14], [15], conductometry [12], [15], [19], light scattering [20], [21], [22], and voltammetry [23]. In our laboratory we have used the liquid-phase polymer-based retention (LPR) technique [24]. This technique combines the use of water-soluble polymers with ultrafiltration membranes, which separate low molecular mass species as free ions from high molecular mass compounds as the precursor polymer and polymer–metal complexes (PMC). It is assumed that the only separation mechanism is the size exclusion by the ultrafiltration membrane. The LPR technique has important technological applications [25], [26]. The projection of its use has the great challenge of increasing the selectivity of the water-soluble polymer (WSP) used towards binding specific metal ions [3], [27], [28], [29], [30].
The LPR technique has demonstrated to be an excellent tool to quantitatively study the polymer–metal ion interaction [31], [32], [33]. By application of the LPR technique by the washing method a retention profile is obtained. This corresponds to a plot of retention (R) versus the filtration factor, (F) where R is defined as the ratio between the amount of metal ions in the ultrafiltration cell at every instant and the initial amount of metal ions, and F is defined as the ratio between the filtrate volume Vf and the volume in the ultrafiltration cell, Vo. This retention profile gives information about the affinity of the metal ions to bind the polyelectrolytes.
Despite the advance in the study of the polyelectrolyte–metal ion interaction through the LPR technique, there are still non-solved questions such as how to differentiate purely electrostatic interactions from others involving coordinative bonds. In this paper, we analyze the nature of the interactions of polyelectrolytes bearing carboxylate and sulfonate groups on their structure with Cu(II) and Ni(II) by means of the LPR technique.
Section snippets
Materials
The commercial polymers poly(acrylic acid) (PAA) and poly(vinylsulfonic acid) (PVS), (both Aldrich) were purified and fractionated by ultrafiltration. PAA has, according to Aldrich, Mw=250,000, therefore a fraction between 100,000 and 1,000,000 Da was chosen. A fraction between 10,000 and 50,000 Da was studied for PVS. The salts NaNO3, Cu(NO3)2, and Ni(NO3)2 (p.a. grade, Merck), were used as received. The solutions were prepared with twice distilled water whose conductivity was lower than 1 μS cm
Results and discussion
The polyelectrolytes PAA and PVS are high flexible linear polymers whose functional groups are linked directly to the backbone; they show good chemical and physical stability, very good solubility in water, and a high capacity to incorporate metal ions due to the high local concentration of functional groups. They also exhibit specific properties: PVS is a strong polyelectrolyte, and is deprotonated in a wide range of pH, while PAA is a weak polyelectrolyte that deprotonates from pH 3.5 to pH
Conclusions
The interactions between the water-soluble polyelectrolytes, poly(acrylic acid), PAA and poly(vinyl sulfonic acid), PVS, and Cu(II) and Ni(II) were studied by ultrafiltration. On the basis of the differences in charge densities of the polyelectrolytes and in the concentration of single salt present in solution, it was pointed out that the interaction PAA–Ni(II) is basically electrostatic, and charge related formation constants for the systems Ni(II)–PAA, Ni(II)–PVS, and Cu(II)–PVS were found to
Acknowledgements
The authors thank FONDECYT (Grant No. 1030669 and No. 1020198) and the Dirección de Investigación of the Universidad Austral de Chile (Grant No. S-200126) for their financial support. N.Sch. thanks CONICYT for providing PhD fellowship.
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